Fuel cells generate power that can potentially be used in a variety of different applications. Fuel cells may eventually replace internal combustion engines in automobiles and trucks. Fuel cells may also power homes and businesses. There are many different types of fuel cells. For example, a solid-polymer-electrolyte membrane (PEM) fuel cell includes a membrane that is sandwiched between an anode and a cathode. To produce electricity through an electrochemical reaction, hydrogen (H2) is supplied to the anode and air or oxygen (O2) is supplied to the cathode.
In a first half-cell reaction, dissociation of the hydrogen (H2) at the anode generates hydrogen protons (H+) and electrons (e−). Because the membrane is proton conductive and dielectric, the protons are transported through the membrane. The electrons flow through an electrical load that is connected across the electrodes. In a second half-cell reaction, oxygen (O2) at the cathode reacts with protons (H+) and electrons (e−) are taken up to form water (H2O).
During development of fuel cell systems for specific applications such as vehicles, an operator such as an engineer or technician monitors the starting and operation of the fuel cell stack under controlled conditions. The operator manually controls stack operating parameters such as reactant and cooling flow rates and temperatures and a current load applied to the fuel cell stack. More recently, fuel cell stack testing equipment has automated some testing of fuel cell stacks. The operator is not required to be present when the fuel cell stack is operated in medium to high output modes. When the fuel cell stack is tested at lower output modes, the operator should be present during operation.
There is an optimum amount of reactant gas that should be supplied to the fuel cell stack to support a desired current load. Usually control systems deliver an additional amount of reactant gas to the fuel cell stack to account for system leaks and inefficiencies and to allow the fuel cell stack to perform more smoothly. When fuel cell stacks are installed as powerplants in vehicles, the extra reactants that are supplied to the fuel cell stack require larger fuel cell stack components and decrease the efficiency of the fuel cell stack. These factors lead to greater production costs due to the larger components. Operating costs are also increased due to the additional reactant gas. Testing and research is currently being performed on fuel cell stacks to lower the additional reactants that are supplied to the fuel cell stack.
During testing to determine the precise amount of reactants that are required, the fuel cell stack is often operated in a low or relatively weak performance mode. Sometimes the fuel cell voltage output drops to zero or reverses potential, both of which can cause damage to the MEA of the fuel cell stack. The operator usually monitors the fuel cell stack during this type of testing even if automated testing equipment is available. When the fuel cell voltage drops quickly or reverses potential, the operator must act quickly to shut down the fuel cell stack to prevent damage.
There are other situations where precise control of the fuel cell stack is needed. The fuel cell stack can be started easily at or near room temperature. At these temperatures, the fuel cell stack provides full rated current relatively quickly, typically within 5-8 seconds following the delivery of reactants. When the temperature of the fuel cell stack is near or below freezing, care should be taken when starting the fuel cell stack to avoid damage. When the current load increases before thawing, individual fuel cells within the fuel cell stack may fail.
The failure of the individual fuel cells prevents the fuel cell stack from supplying the full rated current load. The residual frozen water slows reaction rates because fewer active sites are available for reaction. Rapid increases in the current load cannot be handled by the frozen fuel cell stack.